An aluminum electrolytic capacitor has an advantageous feature such that it has a reduced size and a large electrostatic capacity, and is widely used in low frequency filter and by-pass. The aluminum electrolytic capacitor generally has a structure such that an anode foil and a cathode foil are spirally wound via a separator which prevents the occurrence of short-circuiting between the anode and the cathode, and placed and be sealed up in a casing (see FIG. 1 and FIG. 2). As the anode foil, aluminum having an insulating oxide film formed thereon as a dielectric layer is used, and as the cathode foil, an aluminum foil treated by etching is generally used. The separator disposed between the anode and the cathode is impregnated with an electrolytic solution which functions as an actual cathode.
Among the properties of the electrolytic solution, electric conductivity directly affects the electrolytic capacitor in respect of energy loss and impedance characteristics, and therefore an electrolytic solution having high electric conductivity is being vigorously developed. For example, electrolytic solutions comprising a quaternary ammonium salt (e.g., Japanese Prov. Patent Publication Nos. 145715/1987 and 145713/1987) or a quaternary amidinium salt (e.g., WO95/15572 pamphlet and Japanese Prov. Patent Publication No. 283379/1997) of phthalic acid or maleic acid dissolved in an aprotic solvent such as γ-butyrolactone, have been proposed. However, these electrolytic solutions do not have satisfactory ionic mobility, and further they unsatisfactorily achieve anodization of the anode aluminum, and hence they can be generally used only in capacitors having a rated voltage of 35 V or less.
For removing the disadvantages, an electrolytic solution for electrolytic capacitor having high electric conductivity and excellent heat stability as well as higher withstand voltage, and an electrolytic capacitor having lower impedance and excellent heat stability as well as higher withstand voltage are desired.
The present inventors have previously found that an electrolytic solution for electrolytic capacitor containing tetrafluoroaluminate ions satisfies the requirements (Japanese Patent Application No. 2002-135387). However, the electrolytic capacitor using this electrolytic solution has a problem in that it hardly maintains the initial properties including high electric conductivity, excellent heat stability and high withstand voltage for a long term. In addition, the electrolytic capacitor using the electrolytic solution has a problem in that a current which frequently flows after a direct voltage is applied to the electrolytic capacitor for a certain time, i.e., a leakage current is large.
In this connection, the present inventors have made studies on the moisture in the electrolytic solution. The moisture in the electrolytic solution is not considered to cause a problem in a conventional aluminum electrolytic capacitor using an electrolytic solution comprising an electrolyte such as a quaternary ammonium salt or a quaternary amidinium salt of phthalic acid or maleic acid dissolved in a polar aprotic solvent such as γ-butyrolactone, and for example, when the electrolytic solution has a moisture content of about 3% by weight, there is no problem in the practical use.
However, from the studies made by the present inventors, it has been found that, in the aluminum electrolytic capacitor using an onium salt of fluorine-containing anion as an electrolyte, the moisture in the electrolytic solution considerably affects the performance of the capacitor. The reason for this is presumed that the moisture in the electrolytic solution remarkably affects the state of surface of the aluminum electrode. They have found that, by restricting the moisture content of the electrolytic solution, there can be provided an electrolytic capacitor which solves the above problems, particularly which is improved in the leakage current properties and life properties of the capacitor and can be used stably for a long term, and thus the present invention has been completed.
Further, with respect to the two types of aluminum electrolytic capacitors each using an electrolytic solution containing tetrafluoroaluminate wherein one markedly deteriorates in performance and another does not deteriorate in performance during the use of them for a long term, the present inventors have made close studies on the surface of the respective electrodes. As a result, they have found that, in the electrolytic capacitor which markedly deteriorates in performance, aluminum in the cathode is fluorinated.
In the conventional aluminum electrolytic capacitor using an electrolytic solution comprising an electrolyte such as a quaternary ammonium salt or a quaternary amidinium salt of phthalic acid or maleic acid dissolved in a polar aprotic solvent such as γ-butyrolactone, the cathode surface is not fluorinated, and therefore the fluorination of the cathode surface is found to be a cause of the deterioration of performance. It has been found that an aluminum electrolytic capacitor using a cathode having an Al2p spectrum in the range of 74.0 to 75.8 eV as measured by a method in which the surface of the cathode subjected to a specific accelerated test is analyzed by X-ray photoelectron spectroscopy, is unlikely to suffer fluorination of the cathode and solves the above problems, particularly is improved in the life properties so that it can be used stably for a long term, and thus the present invention has been completed.
In addition, from the studies made by the present inventors, it has been found that, with respect to the electrolytic capacitor using an electrolytic solution which comprises a tetrafluoroaluminate and a solvent, when the total amount of specific compounds contained as impurities in the electrolytic solution is a certain concentration or more, the leakage current of the electrolytic capacitor increases. The reason for this is presumed that these compounds are poor in electrochemical stability and hence a current flows due to the electrochemical reaction of the compounds. Thus, it has been found that, by using an electrolytic solution having a predetermined concentration or less of these compounds, an electrolytic capacitor having improved leakage current properties can be provided, and thus the present invention has been completed.
On the other hand, as methods for preparing an organic onium tetrafluoroaluminate which is useful as an electrolyte for electrochemical device as mentioned above, there are known a method in which an organoaluminum compound and a pyridine-hydrogen fluoride complex are reacted with each other (see, for example, Journal of the American Chemical Society, 1993, vol. 115, p. 3,028); a method in which an organic onium tetrafluoroaluminate and an amine compound are reacted with each other (see, for example, Journal of the American Chemical Society, 1993, vol. 115, p. 3,028); a method in which an organic onium carbonate or hydrogencarbonate and an ammonium tetrafluoroaluminate are reacted with each other (see, for example, Japanese Prov. Patent Publication No. 322760/1999); a method in which an organic onium hydroxide and an ammonium tetrafluoroaluminate are reacted with each other (see, for example, Japanese Prov. Patent Publication No. 322759/1999); and a method in which tetramethylammonium fluoride, aluminum fluoride, hydrogen fluoride and water are reacted with one another (see, for example, Monatshefte fur Chemie, 1975, vol. 106, p. 483).
Further, as methods for synthesizing an organic onium polyfluorometalate, there are known a method in which polyfluorometalate hydroacid and an organic onium halide (see, for example, Journal of Organic Chemistry, 1971, vol. 36, p. 2,371), an organic onium hydroxide (see, for example, Journal of the Chemical Society, Perkin Transactions 2, 1978, vol. 3, p. 254) or an organic onium alkylcarbonate (see, for example, Examined Japanese Patent Application Publication No. 116113/1995) are reacted with each other and a method in which hydrogenfluoride of a quaternary ammonium fluoride is synthesized from a quaternary ammonium salt and a hydrogen fluoride, and then reacted with boron trifluoride (see, for example, Japanese Prov. Patent Publication No. 310555/1999).
However, the method using an organoaluminum has problems in that a spontaneously ignitable substance must be handled, and that the reaction proceeds through pyridinium tetrafluoroaluminate to increase the steps in the method. In the method in which ammonium tetrafluoroaluminate is reacted, the ammonium tetrafluoroaluminate has low solubility, and hence is actually unsuitable for the reaction. Further, the presence of hydroacid of tetrafluoroaluminate (tetrafluoroaluminic acid) is suggested, but complete elucidation has not yet been made, and there is a problem in that it is rarely commercially available.
On the other hand, boron and aluminum belong to the same group of elements. However, unlike the reaction of hydrogenfluoride of an organic onium fluoride and boron trifluoride, the reaction of hydrogenfluoride of an organic onium fluoride and aluminum trifluoride in the presence of excess hydrogen fluoride forms a hexafluoroaluminate having a coordination number of 6 in addition to the tetrafluoroaluminate, and therefore aluminum cannot be applied to this reaction.
Therefore, the development of the method for preparing an organic onium tetrafluoroaluminate which is suitable for the commercial production has been strongly desired.
For solving the above problems, the present inventors have previously invented a method for preparing an organic onium tetrafluoroaluminate which comprises reacting a hydrogen fluoride, an organic onium and aluminum trifluoride (Japanese Patent Application No. 2002-129141).
They have conducted studies with a view toward the developing an improved method for preparing an organic onium tetrafluoroaluminate which has further advantageous commercially. As a result, it has been found that the aluminum trifluoride used in the method can be replaced by hydrogen fluoride or fluorosilicic acid as a source of fluorine and an aluminum compound (excluding aluminum trifluoride) or metallic aluminum as a source of aluminum so that an organic onium salt can be prepared more inexpensively, and thus the present invention has been completed.